This invention relates to a legged mobile robot having at least plural movable legs and specifically to a legged mobile robot capable of walking or otherwise performing movement (locomotion) operations with its movable legs on variable road surfaces. More specifically, it relates to a legged mobile robot capable of walking or otherwise performing movement operations with its movable legs on road surfaces, the, surface states of which are unknown, and also to a legged mobile robot capable of performing adaptive legged movement operations depending on prevailing road surface conditions.
A mechanical apparatus for performing movements simulating the movement of the human being using electrical or magnetic operation is termed a xe2x80x9crobotxe2x80x9d. The etymology of the term robot is to be xe2x80x9cROBOTAxe2x80x9d (slave machine) of the Slavic language. The robots started to be used widely in this country towards the end of sixtieth. Most of the robots used were industrial robots, such as manipulators or transporting robots, aimed at automation or unmanned operations in plants.
The standstill type robot, installed and used at a fixed place, such as armed robots, are in operation only in a stationary or local working space, such as for assembling or sorting of component parts. On the other hand, the mobile robots are not limited as to working space and are movable on a preset or undefined path in an unrestricted fashion to perform operations to take the place of human operators or to offer variegated services to take the place of the human being, dogs or other living organisms. The legged mobile robots, while being unstable and difficult to control as to its orientation or walking, as compared to crawler or tire type robots, are superior in climbing up and down a ladder or a staircase, in riding over obstacles or walking or running flexibly on a leveled or unleveled terrain.
In recent years, researches and development in legged mobile robots, including pet type robots, simulating the bodily mechanism or movements of animals, such as quadruples, e.g., dogs or cats, or so-called humanoid robots, simulating the bodily mechanism or movements of animals erected and walking on feet, such as human being, are progressing, and expectations may be made of practical utilization of these robot types.
The significance of researching and developing the legged mobile robots, termed humanoid robots, may be grasped from, for example, the following two points:
One is the point from the human science. That is, the mechanism of the natural movements of the human being, including the walking, may be clarified scientifically through the process of creating a robot having the structure of legs and/or feet, similar to that of the human being, and of devising its controlling method to simulate the walking movements of the human being. The results of these researches can possibly be fed back to variable fields of researches dealing with the movement mechanism of the human being, such as the fields of human engineering, rehabilitation engineering or sports science.
The other is the development of practically useful robots, supporting the human life as a partner, that is supporting the human activities in various aspects of the everyday life such as in living environment. With the robots of this type, it is necessary to learn the method for adaptation to the human being with different personalities and to different environments as the robots are taught from the human being to make further progress in functional aspects. It may be surmised that, if the, robots are xe2x80x9chumanoidxe2x80x9d, that is of the same form or structure as the human being, the communication between the human being and the robot will be smoother.
For example, if the robot is to be taught as to a technique of passing through a chamber as he evades an obstacle that he should not tramp, it will be far easier for a user (operator) to teach it in case the robot is the robot walking on two legs than in case the robot is of the completely different structure from the teacher as in the case of the crawler or four-legged robot. Similarly, such a robot would learn more easily. In this respect, see xe2x80x9cControl of Robots Walking on Two Feetxe2x80x9d of Automobile Technical Association of Japan, Kanto Branch,  less than KOSO greater than , No. 25, April 1996.
The working space and the living space of the human being is mostly formed to suit to the bodily mechanism or pattern of behavior of the human being erected and walking on two legs. Stated differently, there exist too many obstacles in the human living space for the mechanical systems of the current technical level having wheels or other driving devices as movement means. In order for the mechanical system, that is the robot, to support or act for the human being in variable human operations and to adapt itself more deeply to the living space of the human being, it is desirable that the movable range of the robot is approximately the same as that of the human being. This accounts for the expectations made of development of practically useful legged mobile robot. The human type configuration of the robot may be said to be indispensable in order for the robot to adapt itself more readily to the human living environment.
A number of proposals have already been made for achieving orientation control or stabilized walking of the robot of the type performing legged movement using two legs. The stable xe2x80x9cwalkingxe2x80x9d herein means xe2x80x9cmovement on feet without tumbling downxe2x80x9d.
The stabilized orientation control for a robot is crucial in evading its leveling. The reason is that leveling means interruption of the operation being performed by the robot and considerable time and labor is needed until the robot erects itself from its leveled state to re-initiate the operation, and that, more importantly, the leveling tends to inflict a fatal damage not only to the robot but also the object against which has impinged the robot. Thus, stabilized orientation control and prevention of leveling during walking is most crucial in the engineering and development of the legged mobile robot.
During walking, the force of gravity, the force of inertia and the moment thereof act from the walking system to the road surface due to the force of gravity and the acceleration produced by the walking movement. According to the xe2x80x9cd""Alembert""s principlexe2x80x9d, these are counterbalanced by the reaction of the floor as the reaction from the floor surface to the walking system, that is the reactive moment of the floor. As a conclusion of mechanical inferences, a point of zero pitch axis and roll axis moment, that is a zero moment point (ZMP), exists on or inside a side of a supporting polygon delimited by the contact point of the foot sole with the ground and the road surface.
The majority of proposals so far made for preventing the leveling of the legged mobile robot use this ZMP as the criterium in judging the stability in walking. The generation of a pattern of walking on two legs, derived from the ZMP criterium, has advantages that the ground touching point of the foot sole can be previously set such that kinematic constraint conditions of the foot end in compliance with the road surface can be considered more readily. Moreover, using the ZMP as the criterium in verifying the stability means that not the force but the trajectory is to be handled as a target value in movement control, thus enhancing the technical feasibility. Meanwhile the concept of the ZMP and using this ZMP as the criterium in judging the walking stability of the robot is stated in Miomir Vukobratovic, xe2x80x9cLegged Locomotion Robotsxe2x80x9d (Ichiro KATO, xe2x80x9cWalking Robots and Artificial Legsxe2x80x9d, published by Nikkan Kogyo Shimbun K K).
However, the legged mobile robot has made but one step from the stage of researches and a number of technical difficulties are still left. For example, the surface states of the road, specifically whether or not the terrain is leveled, or the frictional coefficient, which affects the legged walking significantly, are as yet not tackled sufficiently.
In general, the legged mobile robot performs walking movements as scheduled. In Japanese Laying-Open Patent Publication S-62-97006, there is disclosed a multi-joint walking robot control device in which pre-stored walking pattern data are used to simplify a control program and in which the respective data of the walking pattern can be linked intimately. However, in actual walking, it is a frequent occurrence that relative movement between the road surface and the foot sole, such as slip, is produced due to the relation with respect to the road surface, or other factors, such as wind from a lateral side or an external force, or a conflict with an unknown obstacle.
The point of the center of gravity is higher and the ZMP stability area defining the walking is narrower with the two-legged robot, such as humanoid, than with a four-legged robot. Thus, the problem of variations in orientation due to changes in the road surface state is particularly critical in the two-legged mobile robot.
FIGS. 1 to 4 show how a legged mobile robot 2000 performs walking on two legs. As shown therein, the legged mobile robot 2000 is able to realize walking on two legs by repeating walking periods each consisting in the following movement time intervals:
(1) the time period in which the robot is supported by a left leg 2002, with the right leg 2001 lifted (FIG. 1);
(2) the time period during which a right leg 2003 of the robot contacts the floor, with the both legs thus being set on the floor (FIG. 2);
(3) the time period during which the left leg 2002 is lifted, with the robot being supported only on the right leg 2001 (FIG. 3); and
(4) the time period during which the left leg 2004 of the robot contacts the floor, with the both legs thus being set on the floor (FIG. 4).
In robot walking control, a target trajectory of the lower legs is previously measured and the scheduled trajectory is corrected in each of the above time periods. For example, during the time period when the both legs support the robot, correction of the trajectory of the lower legs with the road surface information is halted and the total correction quantity with respect to the scheduled trajectory is used to correct the waist part of the robot to a preset value. During the time period when the robot is supported on one leg, a correction trajectory is generated which will restore the relative position between the ankle of the corrected side and the waist part to the scheduled trajectory. A more specified correction is performed by a five-order polynominal so that the position, speed and the acceleration for decreasing the offset with respect to the ZMP will be continuous (see Takanishi, xe2x80x9cControl of Two-Legged Robotxe2x80x9d Automobile Technical Association of Japan, Kanto Branch,  less than KOSO greater than , No. 25, April 1996.
Taking the time period of support on a sole leg shown in FIG. 1, the case in which the road surface is relatively moved (slipped) with respect to the foot sole. In the two-legged mobile robot 2000, it is expected from the trajectory scheming that, during the time period when the erected sole leg contacts the road surface, no relative movement (slip) of the erected leg occurs with respect to the road surface.
FIG. 5 shows the state in which the relative movement (slip) of the left leg 2004 has occurred with respect to the road surface during the time period when the erected left leg 2002 contacts the road surface face. As shown, the left leg 2004 of the robot 2000 is relatively moved by xcex94X in the proceeding direction (roll axis direction) and xcex94Y in the direction perpendicular thereto (pitch axis direction). That is, since the left leg 2004 is scheduled to contact the road surface completely; there is produced deviation (xcex94X, xcex94Y) between the scheduled or intended trajectory and the actual trajectory of the robot.
During the time period when the erected sole leg contacts the road surface, it is necessary to generate a corrected trajectory so that the relative positions of the waist and the wrinkles of the feet corrected will be restored to the scheduled trajectory. However, it is extremely difficult to measure the relative movement with respect to the road surface (slip) to a correct value, using a detection system, loaded as a standard item on a robot, such as acceleration sensor or visual perception by a camera. The result is that the robot is unable to correct the trajectory accurately and promptly, thus possibly leading to the overlooked relative movements until such time the difference from the scheduled trajectory is increased to s considerable extent. Such difference between the scheduled trajectory and the actual trajectory may lead to failure in stabilized orientation control or collision with an obstacle which should have been evaded, thus occasionally resulting in leveling down of the robot or in the destruction of the robot and its near-by objects.
FIG. 6 shows the state in which, during the time period when the erected left leg 2002 contacts the road surface, the relative movement (slip) of the left leg 2004 is produced, in which the left leg 2004 is rotated around a normal of the road surface or the normal line drawn from the road surface (yaw axis). As shown, the left leg 2004 of the robot 2000 has been rotated through xcex94xcex8 relative to the normal line drawn from the road surface. That is, since the left leg 2004 is about to contact the road surface completely, a rotational offset of xcex94xcex8 is produced between the scheduled or intended trajectory and the actual trajectory of the robot 2000.
Such rotational offset is extremely difficult to detect to an accurate value, using a detection system loaded as a standard item on the robot, such as gyro sensor or visual perception by a camera. Such offsetting around a normal of the road surface affects the proceeding direction of the robot, the continued walking operation may lead to significant trajectory deviation.
In general, when relative movements are produced in the respective legs 2003, 2004, these movements are frequently produced on synthesis of the relative movements within a plane parallel to the road surface shown in FIG. 5 and those around the normal line drawn from the road surface shown in FIG. 6, thus enhancing the adverse effects on the walking.
In this consideration, axle force sensors for sensing the force acting on the legs of the robot may be used to measure the force of reaction from the road surface to eliminate the effect of the relative movements (slip). It is however extremely difficult to remove the adverse effect completely.
It may be contemplated to measure the orientation of the robot itself by a gyro sensor or an acceleration sensor, or with a visual sensor, such as camera, to measure the effect of the relative movement (slip) or to feed back the result to the subsequent trajectory schedule. However, these measures frequently lead to problems in measurement precision or response speed. In case of a robot destined to walk on variegated road surfaces, such walking that permits of no relative movement (slip) whatsoever is impossible in reality, while an approach to slip elimination is not meritorious from the perspective of the movement speed.
If the notion that, when the legged mobile robot walks, relative movements (slips) are necessarily produced in the legs, is maintained, such a walking process may be contemplated in which relative movements are tolerated and the slip is quantified, analyzed and managed while the slip is exploited positively.
When a person walks, relative movements (slip) may be frequently produced between the road surface and the foot sole. Thus, for realization of variegated walking patterns in the legged mobile robot, it is necessary to take the relative movements (slip) positively into the trajectory schedule and further to execute trajectory correction. To this end, it is crucial in a robot performing operations with its legs that the relative movement (slip) of the respective legs with respect to the road surfaced can be recognized or measured.
The majority of the sensors provided on the feet of the conventional legged mobile robots measure the force of reaction from the road surface or the distance to the road surface. Stated differently, the equipment for measuring or recognizing the physical quantity corresponding to the relative movement (slip) of the respective legs with respect to the road surface, such as movement in a plane parallel to the road surface or movements around the normal line drawn from the road surface, has not been applied so far to the robot. In realizing the so-called foot sole sensation on the legged mobile robot, it may be contemplated to be crucial to detect and quantify the relative movement (slip) of the respective legs with respect to the road surface.
It is an object of the present invention to provide a legged mobile robot capable of walking on variable road surfaces and of performing movement operations on feet with movable legs.
It is another object of the present invention to provide a legged mobile robot capable of performing walking or other operations on feet with movable legs on variable road surfaces.
It is still another object of the present invention to provide a legged mobile robot capable of performing adaptive movement operation on feet depending on the prevailing road surface states.
It is still another object of the present invention to provide a legged mobile robot capable of performing adaptive operational control, such as correction of scheduled trajectories, as it recognizes or measures the relative movement (slip) of the respective legs with respect to the road surface.
It is yet another object of the present invention to provide a legged mobile robot capable of performing adaptive operational control of the legged mobile robot, such as by recognizing or measuring the relative movement of the respective legs with respect to the road surface to correct the scheduled trajectory.
For accomplishing these objects, the present invention provides a legged mobile robot having at least a plurality of movable legs, including a relative movement measurement sensor arranged on the foot sole of each movable leg for measuring the amounts of relative movements between the foot part and the road surface; and a controller for controlling the movements of the legged mobile robot based on the amounts of relative movements between the foot part and the road surface as measured by the relative movement measurement sensor.
The relative movement measurement sensor preferably measures the amount of the relative movement of the foot part relative to a direction parallel to the road surface and/or the amount of the relative movement of the foot part around a normal of the road surface.
The relative movement measurement sensor preferably includes a rotationally mounted ball having its surface partially exposed to outside the foot sole, a first rotation unit for detecting the rotation in a first direction of the ball, a second rotation unit for detecting the rotation in a second direction of the ball and a calculation unit for calculating the amount of the relative movement of the foot part relative to the road surface in the first and second directions.
Alternatively, the relative movement measurement sensor includes imaging means for imaging a ground touching surface of the foot part, image processing means for comparing images photographed by the imaging means at a preset time interval and calculating means for calculating the amount of the relative movement relative to the road surface of the foot part at the preset time interval based on the results of image comparison by the image processing means.
If the relative movement measurement sensor is able to measure the amount of the relative movement of the foot part in a direction parallel to the road surface, the amount of the relative movement around a normal of the road surface may be calculated by chronologically joining the amounts of the relative movement in the direct parallel to the road surface of the foot part as measured at present time intervals.
Two or more relative movement measurement sensors may be provided on each foot part at a spacing from one another, in which case the measured results of the relative movement measurement sensors are compared to one another to calculate the amount of the relative movement of the foot part around a normal of the road surface.
The controller may halt the movement on the road surface responsive to the amount of relative movement between the foot part and the road surface exceeding a preset threshold during movement on the road surface.
After halting the movement operation, decision may be made as to whether or not the movement operation can further be continued. A preset assistance request operation may be executed when it is verified that the movement operation cannot be further continued. The assistance request may be made by uttering a speech or calling a near-by user by exploiting wireless communication. Alternatively, the road surface state may be searched and a behavior may be executed responsive to the searched results.
The controller may also transfer to a safe movement operation pattern for a road surface responsive to the amount of the relative movement between the foot part and the road surface exceeding a preset threshold value during movement on the road surface. The safe movement operation pattern herein may include enlarging the incidence angle of the foot part touching the road surface, lowering the movement speed, diminishing the footstep of each movable leg and correcting the ZMP trajectory.
The controller may also transfer to the movement operation under searching the road surface responsive to responsive to the amount of the relative movement between the foot part and the road surface exceeding a preset threshold value during walking on the road surface.
The present invention also includes a method for controlling a legged mobile robot having at least a plurality of movable legs, including a measurement step of measuring the amounts of relative movements between the foot part and the road surface when the respective movable legs touch the road surface, and a controlling step of controlling the movements of the legged mobile robot based on the amounts of relative movements between the foot part and the road surface as measured.
According to the present invention, the measurement step measures the amount of the relative movement of the foot part relative to a direction parallel to the road surface and/or the amount of the relative movement of the foot part around a normal of the road surface.
If the measurement unit is to measure the amount of relative movement of the foot part in a direction parallel to the road surface, there may be provided a calculation step of calculating the amount of the relative movement around a normal of the road surface by chronologically joining the amounts of the relative movement in the direction parallel to the road surface of the foot part as measured in the measurement step at present time intervals.
Alternatively, the measurement step may include a calculating step of measuring the amount of the relative movement between the foot part and the road surface at two or more spaced apart points and comparing the amounts of the relative movement measured at two or more points to one another to calculate the amount of the relative movement of the foot part relative to a normal line from the road surface.
In the controlling step, the movement on the road surface may be halted responsive to the amount of the relative movement between the foot part and the road surface exceeding a preset threshold value during movement on the road surface.
It may also be verified, after halting the movement, whether or not the movement operations can be continued further. The preset assistance requesting operation may also be executed if continuation of the movement operation is found to be impossible. The assistance request may include uttering a speech or calling a near-by user by exploiting wireless communication.
The controlling step may halt the movement on the road surface responsive to the amount of the relative movement between the foot part and the road surface exceeding a preset threshold value, during movement on the road surface, to search the state of the road surface. The controlling step may also execute a behavior responsive to the searched result.
The controlling step may also transfer to a safe movement operation pattern for the road surface responsive to the amount of the relative movement between the foot part and road surface exceeding a present threshold value during movement on the road surface 26. The safe movement operation pattern may at least be one of enlarging the incidence angle of the foot part touching the road surface, lowering the movement speed, diminishing the footstep of each movable leg and correcting the ZMP trajectory.
The controlling step may also transfer to the movement operations responsive to the amount of the relative movement between the foot part and the road surface exceeding a preset threshold value during walking on the road surface. The controller may execute the processing of selecting a foot sole suited to the road surface responsive to the amount of the relative movement between the foot part ands the road surface exceeding a preset threshold value during walking on the road surface.
The present invention also provides a relative movement measurement sensor for a legged mobile robot for measuring the amount of relative movement between a foot part and the road surface, the sensor being applicable to a foot part of a legged mobile robot having at least a plurality of movable legs, in which the relative movement measurement sensor includes a rotationally mounted ball having its surface partially exposed to outside the foot sole, a first rotation unit for detecting the rotation in a first direction of the ball, a second rotation unit for detecting the rotation in a second direction of the ball and a calculation unit for calculating the amount of the relative movement of the foot part relative to the road surface in the first and second directions.
The present invention also provides a relative movement measurement sensor for a legged mobile robot for measuring the amount of relative movement between a foot part and the road surface, the sensor being applicable to a foot part of a legged mobile robot having at least a plurality of movable legs, in which the relative movement measurement sensor, imaging means for imaging a ground touching surface of the foot part, image processing means for comparing images photographed by the imaging means at a preset time interval and calculating means for calculating the amount of the relative movement relative to the road surface of the foot part at the preset time interval based on the results of image comparison by the image processing means.
The robot according to the present invention is able to perform variable movement operations on legs, such as walking, using plural movable legs. For example, walking on two-legs may be realized by left and right legs repeatedly performing the operation of supporting the robot on one of the left and, right legs and supporting the robot on both legs in accordance with an intended or scheduled lower limb trajectory scheme. During the time periods when the robot is supported on one leg or on both legs, the scheduled trajectory may be corrected, or a corrected trajectory may be generated, to restore the scheduled trajectory depending on the road surface information or the relative positions between the ankle and waist parts.
During the operations of the robot, erected on legs, it is a frequent occurrence that relative movement, such as slipping, may occur between the foot part erected or touching the ground and the road surface. Such relative movement may cause deviation of the actual trajectory from the scheduled trajectory of the robot to cause leveling down or collision of the robot against an obstacle. It is therefore necessary to measure the amount of relative movement between the foot part and the road surface accurately and to correct the scheduled trajectory based on this amount of relative movement by way of performing adaptive control of the operations being executed by the robot.
In view of the above object, with the legged mobile robot of the present invention, the foot part of each movable leg (foot flat or the foot sole) is provided with a relative movement measurement sensor between the road surface and the leg touching the ground, in addition to the road surface touching sensor for confirming the road touching state between the foot part and the road surface, thus assuring adaptive control even in case the deviation is produced between the intended or scheduled trajectory.
Other objects, features and advantages of the present invention will become more apparent from reading the embodiments of the present invention as shown in the drawings.